The wheel axles of a truck trailer are invariably connected to the trailer bed or frame by way of a suspension which cushions the trailer against road impacts. The suspension for each end of the axle usually includes a relatively long multiple-leaf leaf spring whose opposite ends are pivotally connected via bushings to brakes secured at spaced apart locations along the length of a trailer side frame member. The wheel axle extends transversely between a pair of such leaf springs being connected near its opposite ends to those springs midway along their lengths.
As a general rule, the axle is connected to each spring by a saddle clamp assembly comprising a pair of U-bolts which engage over the midportion of the spring and extend down the sides of the spring and down along the sides of the axle through a pair of openings formed in the opposite sides of a cast metal saddle block which engages under the axle. The threaded ends of the bolts extend through the block and suitable nuts are turned down onto those bolt ends to draw the bolts toward the block and thereby clamp the axle to the spring. In a variant of the aforesaid construction, the U-bolts extend upwardly around the sides of the axle past the sides of the spring and are clamped to a saddle block engaged on the spring by nuts turned down onto the U-bolts.
Usually, also a torque arm is connected between the block and the frame bracket supporting the forward end of the spring to strengthen the suspension and prevent the axle and block from cocking relative to the center of the spring.
A problem arises with these conventional trailer suspensions, however. The various leaves in each leaf spring are normally connected together at their centers by a center bolt which extends through holes in the leaves. Consequently the spring as a whole is weakest at its center adjacent those holes. Because of age and rough usage, a leaf spring on a particular trailer may break at the central location adjacent the center bolt and between the two U-bolts where it suffers the most stress in use. When this occurs, the saddle block which is attached to the torque arm and trailer frame is allowed to swing free from the axle so that when the trailer brakes are applied, there is nothing holding the axle to the frame on the side of the trailer where the spring break occurs. Consequently, the axle is free to skew relative to the frame, which skewing may cause the saddle clamp assembly connecting the other end of the axle to its spring to snap. Resultantly, the entire axle and wheels may separate completely from the trailer causing a serious accident.
Also, if one of the suspension U-bolts should break, say the rear U-bolt on a rearmost suspension, and the trailer is backed up against an obstacle or over a relatively high bump, the impact against the wheel at that side of the trailer may cause that end of the axle to twist about its axis and break the remaining U-bolt holding that end of the axle to its spring giving rise to the same problem described above.
The potential for accident is especially serious if the wheels on a particular trailer are in tandem or pairs because if the forward wheel axle of a pair breaks away, the rearward axle in the pair may also be separated from the frame because of the forward axle and wheels smashing or being driven into it. Due to these various problems affecting prior truck trailer suspensions, a surprising number of road accidents occur every year, particularly when such trailer trucks are driven off-road or on secondary roads.